To meet the growing need for ever increasing digital data bandwidth for new subscriber services (e.g., high-data-rate internet service, video telephony, and high definition television) telephone companies are looking towards the use of very-high-speed digital subscriber lines (VDSL). VDSL provides a means to carry such data into the home or business over the existing copper wires used by plain old telephone service (POTS). VDSL carries digital data at bit rates reaching 52 Mbps using carrierless AM/PM (CAP) modulation, which is related to quadrature amplitude modulation (QAM). Modulation contellations can reach 256 symbol points with symbol rates up to 6480 Kbaud. The CAP modulation constellations look the same as the corresponding baseband QAM constellations. The signals are carried within the spectral band up to 30 MHz, which is above the frequency band used by POTS on the same wire pair. Due to the high loss of the copper telephone wires at these frequencies, the VDSL signals are carried on the telephone wires only over the "last mile" (i.e., the last segment of copper wire between a central office and the user's premises).
Within the spectral band of 0.5 MHz to 30 MHz, the VDSL transmissions over the telephone wires are subject to radio frequency interference (RFI) from AM radio broadcasts and amateur radio transmissions, which can be picked up by the telephone wires acting as receiving antenna. This interference can corrupt the received and demodulated CAP/QAM signals, causing symbol and bit errors in the received data. The AM broadcast signals remain indefinitely at the same frequencies in any neighborhood within range of the local AM radio transmitters. The amateur radio signals, however, will vary in frequency throughout the day as the operator tunes his transmitter to optimize the HF-band skywave propagation conditions over the communication links he is attempting to maintain. There are several narrow spectral bands allocated to amateur radio within the 0.5 MHz to 30 MHz band carried by VDSL transmission lines. A VDSL transmission line will normally be interfered with only by amateur radio transmitters within a few hundred feet of the transmission line.
Narrowband interferers, such as AM radio broadcasts and amateur radio signals, are not the only interferers that corrupt the CAP/QAM signals. Wideband noise that extends across a large portion of the VDSL spectrum may also interfere, causing undesirable changes in the carrier-to-noise ratio (CNR) and the bit error rate (BER). Wideband noise can be caused, for example, by electrical machinery, internal combustion engines (e.g., lawn mowers) or fluorescent lights operating in the immediate vicinity of a cable drop to the premises. Narrowband interferers tend to be more predictable and hence somewhat easier to avoid, for example, by detecting the frequency of the interferer and subsequent adjustment of the CAP/QAM VDSL signal for avoidance thereof. Wideband interferers, on the other hand, are somewhat more random, and cannot generally be accounted for prior to data transmission and are therefore more difficult to avoid. Especially difficult is detection and characterization of the interferers without interruption of the data signal for analysis.
Thus, what is needed is a method and apparatus for recognizing the difference between narrowband interferers and wideband noise sources without relying upon spectrum analysis of the received signal except to locate a new transmission band. This is accomplished by the present invention whereby a determination is made of the presence or absence of a narrowband or continuous wave (CW) interferer, and in many cases inferring its carrier frequency without the need for interrupting the transmitted data stream. The transmitted data stream is only interrupted when an inteferer is detected and a spectrum analysis is needed to find a new transmission band. The interferer presence determination can generally be accomplished when the interferer level is low enough that the presence of the interferer cannot be inferred by visual examination of the constellation plot of the demodulated signal. When a narrowband signal is present, the shape of the symbol clusters in the constellation plot changes from a centrally peaked distribution, characteristic of a signal in Gaussian noise, to a ring as shown in FIGS. 1A and 1B. The present invention recognizes this transition automatically and at low interference levels where the transistion to a ring is not yet visually apparent. Adjustment of the transmitted symbol rate, bandwidth and carrier frequency is provided in order to avoid the troublesome interferer and other interferers that may be present in the signal environment.